Specific types of physiological condition monitors are capable of monitoring specific types of physiological information. For example, one specific type of physiological condition monitor may be capable of monitoring a person's respiration activity. Other specific types of physiological condition monitors may be capable of monitoring cardiac activity, or blood oxygenation levels, or blood glucose levels, or movement of a body, or position orientation of a body, or other similar physiological conditions. A physiological condition monitor usually comprises one or more appropriate sensors coupled to the body of the person whose physiological conditions are to be measured.
In the case of sensors for detecting respiration activity or cardiac activity, the sensors are capable of sensing changes in pressure (or changes in other types of physical parameters) that are caused by the person's breathing and cardiac activity. Physiological condition monitors measure and record waveform signals received from the sensors. Electrocardiogram (ECG) waveform signals are the most commonly used waveforms for measuring a person's cardiac activity. Respiration waveform signals are used to measure a person's breathing rate and other types of information concerning respiration.
In the case of sensors for detecting blood oxygenation levels or blood glucose levels, the sensors are capable of sensing changes in the level of oxygen in the blood or changes in the level of glucose in the blood as those changes occur in the person's blood.
The present invention is capable of providing a significant reduction in the power consumption of any type of physiological condition monitor. For purposes of illustration, however, the present invention will first be described with reference to physiological condition monitors that are capable of monitoring respiration and cardiac activity. It is understood, however, that the present invention is not limited to use in respiration monitors or in cardiac activity monitors.
Low heart rate is referred to as bradycardia. Cessation of respiration is referred to as apnea. When a person exhibits apnea or bradycardia a life threatening condition very likely exists. Physiological condition monitors that are capable of continuously monitoring a person's respiration and cardiac activity are extremely useful for quickly detecting apnea or bradycardia. Such physiological condition monitors are also useful for quickly detecting other abnormal conditions such as a high heart rate (known as tachycardia) or a very slow breathing rate or a very high breathing rate.
Infants who are susceptible to sudden infant death syndrome are known to exhibit apnea and bradycardia. Physiological condition monitors that are capable of continually monitoring respiration and cardiac activity are particularly useful in the early detection of apnea or bradycardia in infants. Most physiological condition monitors are equipped with an alarm system to sound an alert when such conditions are detected.
A physiological condition monitor may be coupled directly to a person who is a patient in a hospital bed. In such an arrangement the waveform signals from the sensors coupled to the patient's body may be sent through wires directly to a detector circuit (and other circuitry) located in a console by the patient's bed. The wires attached to the patient restrict the patient's movements.
In other cases it is more practical to provide a physiological condition monitor located in a belt or harness that is to be worn by the person to be monitored. In this type of monitor the waveform signal information from the sensors is transmitted via a radio frequency transmitter to a radio frequency receiver in a base station unit that is located away from the site of the physiological condition monitor. The base station unit contains circuitry for analyzing and recording the waveform signal information. The base station unit contains circuitry for detecting abnormal conditions in the person's breathing or cardiac activity, such as apnea or bradycardia.
Because of the freedom of movement that this type of monitor provides, it is the preferred type of monitor for monitoring the physiological conditions of infants.
If the data that is acquired by the physiological condition monitor is not transmitted to the base station and recorded there, then the data must be recorded in a memory data storage device located within the physiological condition monitor. To preserve the freedom of movement that is provided by a belt or harness monitor, the memory data storage device within the physiological condition monitor must be battery powered.
One type of battery powered memory data storage device that can be used to record the data is a flash memory data storage system. As will be explained more fully below, the power requirements of prior art flash memory data storage systems have caused them to be inefficient in battery powered applications.
A physiological condition monitor that is capable of recording data in a memory storage device for over an extended period of time is very useful. By recording data over an extended period of time the physiological condition monitor can capture information concerning physiological events that do not occur regularly but occur only sporadically or rarely. A doctor or clinician can use the collected data to identify and evaluate such rare or sporadic physiological events.
For the data recording to have value it must recreate the physiological data in sufficiently fine detail to enable a doctor or clinician to identify and evaluate the physiological events represented by the data. This means that the physiological condition monitor must have a relatively high sampling rate throughout the period of time that the data is being recorded. This means that there will be a large amount of data to store.
There is a direct linear relationship between the amount of data to be stored and the quantity of energy needed to store it. To store a small amount of data requires a correspondingly small amount of electrical power. To store a large amount of data requires a correspondingly large amount of electrical power. In a battery powered memory data storage system in a physiological condition monitor, all of the electrical power must be provided by the battery. In order to collect and record the large amounts of data that are required, it is essential that the electrical power in the battery be conserved and used efficiently.
The present invention is directed toward providing a significant reduction in the power consumption of memory data storage systems used in physiological condition monitors. In particular, the present invention is directed toward providing a significant reduction in the power consumption of battery powered flash memory data storage systems used in physiological condition monitors.
A non-volatile data storage device is one that retains the data stored in it when external power to the device is shut off. One of the earliest non-volatile storage devices was punched paper tape. One of the most recent technologies for storing data in a non-volatile electronic data storage device is called "flash memory." Flash memory is a programable semiconductor memory of a type called "read-mostly" memory. Flash memory is so named because of the speed with which it can be reprogrammed. Flash memory uses an electrical erasing technology that can erase an entire flash memory array in a few seconds at most. Data written to flash memory remains in a non-volatile storage mode until the flash memory is deliberately erased. Flash memory requires a relatively high level of current (and a high level of electrical power to provide that current) when data is being written to the flash memory. A typical value of current required by flash memory when data is being written to the flash memory is sixty milliamps (60 mA).
CompactFlash.TM. memory is a relatively new flash memory data storage system. CompactFlash.TM. is a registered trademark of SanDisk Corporation. CompactFlash.TM. memory is very useful in various types of technological applications and represents a significant advance over other flash memory data storage systems for a number of reasons. In comparison with other flash memory data storage systems, CompactFlash.TM. memory has greater speed, greater durability, and smaller size. It is also packaged in a form that is very compatible with personal computers, especially laptop computers. CompactFlash.TM. memory makes it possible to store several tens of Megabytes of data on a memory card that is no larger than an ordinary matchbook. CompactFlash.TM. memory cards are now being used in digital cameras, in personal data assistants (PDAs), in MP3 audio players, and in other similar electronic data storage devices.
One of the drawbacks of CompactFlash.TM. memory (and of flash memory data storage systems in general) is that its operation requires a relatively high level of current. The greater the speed with which a flash memory data storage system is accessed, the more current it requires for operation. Even at the slowest access speeds, flash memory data storage systems generally require a comparatively large amount of current for operation.
For this reason flash memory data storage systems have not been widely used in battery powered devices for gathering electronic data. This is especially true for battery powered devices that acquire data slowly over a relatively long period of time. The power requirements of a flash memory data storage system in such a device would require continual and frequent replacement of the batteries. In many applications this requirement would make the use of a flash memory data storage system impractical.
It would be advantageous to have a flash memory data storage system in a physiological condition monitor in which the power consumption is reduced compared to the power consumption in prior art flash memory data storage systems. It would also be advantageous that any reduction of the power consumption in such a flash memory data storage system be achieved without a corresponding reduction in the performance level of the flash memory data storage system.